The development of techniques for the manipulation of nucleic acids, the amplification of such nucleic acids when necessary, and the subsequent detection of specific sequences of nucleic acids or amplicons has generated extremely sensitive and nucleic acid sequence specific assays for the diagnosis of disease and/or identification of pathogenic organisms in a test sample.
Amplification of nucleic acids
When necessary, enzymatic amplification of nucleic acid sequences will enhance the ability to detect such nucleic acid sequences. Generally, the currently known amplification schemes can be broadly grouped into two classes based on whether, the enzymatic amplification reactions are driven by continuous cycling of the temperature between the denaturation temperature, the primer annealing temperature, and the amplicon (product of enzymatic amplification of nucleic acid) synthesis temperature, or whether the temperature is kept constant throughout the enzymatic amplification process (isothermal amplification). Typical cycling nucleic acid amplification technologies (thermocycling) are polymerase chain reaction (PCR), and ligase chain reaction (LCR). Specific protocols for such reactions are discussed in, for example, Short Protocols in Molecular Biology, 2.sup.nd Edition, A Compendium of Methods from Current Protocols in Molecular Biology (Eds. Ausubel et al., John Wiley & Sons, New York, 1992) chapter 15. Reactions which are isothermal include: transcription-mediated amplification (TMA), nucleic acid sequence-based amplification (NASBA), and strand displacement amplification (SDA).
U.S. Patent documents which discuss nucleic acid amplification include U.S. Pat. Nos. 4,683,195; 4,683,202; 5,130,238; 4,876,187; 5,030,557; 5,399,491; 5,409,818; 5,485,184; 5,409,818; 5,554,517; 5,437,990 and 5,554,516 (each of which are hereby incorporated by reference in their entirety). It is well known that methods such as those described in these patents permit the amplification and detection of nucleic acids without requiring cloning, and are responsible for the most sensitive assays for nucleic acid sequences. However, it is equally well recognized that along with the sensitivity of detection possible with nucleic acid amplification, the ease of contamination by minute amounts of unwanted exogenous nucleic acid sequences is extremely great. Contamination by unwanted exogenous DNA or RNA nucleic acids is equally likely. The utility of amplification reactions will be enhanced by methods to control the introduction of unwanted exogenous nucleic acids and other contaminants.
Prior to the discovery of thermostable enzymes, methods that used thermocycling were made extremely difficult by the requirement for the addition of fresh enzyme after each denaturation step, since initially the elevated temperatures required for denaturation also inactivated the polymerases. Once thermostable enzymes were discovered, cycling nucleic acid amplification became a far more simplified procedure where the addition of enzyme was only needed at the beginning of the reaction. Thus reaction tubes did not need to be opened and new enzyme did not need to be added during the reaction, allowed for an improvement in efficiency and accuracy as the risk of contamination was reduced, and the cost of enzymes was also reduced. An example of a thermostable enzyme is the polymerase isolated from the organism i Thermophilus aquaticus.
In general, isothermal amplification can require the combined activity of multiple enzyme activities for which no optimal thermostable variants have been described. The initial step of an amplification reaction will usually require denaturation of the nucleic acid target, for example in the TMA reaction, the initial denaturation step is usually .gtoreq.65.degree. C., but can be typically .gtoreq.95.degree. C., and is used when required to remove the secondary structure of the target nucleic acid.
The reaction mixture is then cooled to a lower temperature which allows for primer annealing, and is the optimal reaction temperature for the combined activities of the amplification enzymes. For example, in TMA the enzymes are generally a T7 RNA polymerase and a reverse transcriptase (which includes endogenous RNase H activity). The temperature of the reaction is kept constant through out the subsequent isothermal amplification cycle.
Because of the lack of suitable thermostable enzymes, some isothermal amplifications will generally require the addition of enzymes to the reaction mixture after denaturation at high temperature, and cool-down to a lower temperature. This requirement is inconvenient, and requires the opening of the amplification reaction tube, which introduces a major opportunity for contamination.
Thus, it would be most useful if such reactions could be more easily performed with a reduced risk of contamination by methods which would allow for integrated denaturation and amplification without the need for manual enzyme transfer.
Amplification Buffer and Single Reaction Aliquot of Reagents
Typical reaction protocols require the use of several different buffers, tailored to optimize the activity of the particular enzyme being used at certain steps in the reaction, or for optimal resuspension of reaction components. For example, while a typical PCR 10.times. amplification buffer will contain 500mM KCl and 100 mM Tris HCl, pH 8.4, the concentration of MgCl.sub.2 will depend upon the nucleic acid target sequence and primer set of interest. Reverse transcription buffer (5.times.) typically contains 400mM Tris-Cl, pH 8.2; 400mM KCl and 300 mM MgCl.sub.2, whereas Murine Maloney Leukemia Virus reverse transcriptase buffer (5.times.) typically contains 250 mM Tris-Cl, pH 8.3; 375 mM KCl; 50mM DTT (Dithiothreitol) and 15 mM MgCl.sub.2.
While such reaction buffers can be prepared in bulk from stock chemicals, most commercially available amplification products provide bulk packaged reagents and specific buffers for use with the amplification protocol. For example, commercially available manual amplification assays for detection of clinically significant pathogens (for example Gen-Probe Inc. Chlamydia, and Mycobacterium tuberculosis detection assays) requires several manual manipulations to perform the assay, including dilution of the test sample in a sample dilution buffer (SDB), combination of the diluted sample with amplification reaction reagents such as oligonucleotides and specific oligonucleotide promoter/primers which have been reconstituted in an amplification reconstitution buffer (ARB), and finally, the addition to this reaction mixture of enzymes reconstituted in an enzyme dilution buffer (EDB).
The preparation and use of multiple buffers which requires multiple manual additions to the reaction mixture introduces a greater chance for contamination. It would be most useful to have a single unified buffer which could be used in all phases of an amplification protocol. In particular, with the commercially available TMA assays described above, the requirement for three buffers greatly complicates automation of such a protocol.
Bulk packaging of the enzyme or other reaction components by manufacturers, may require reconstitution of the components in large quantities, and the use of stock amounts of multiple reagents, can be wasteful when less than the maximal number of reactions are to be carried out, as some of these components may be stable for only a short time. This process of reconstitution also requires multiple manipulations by the user of the stock reagents, and aliquoting of individual reaction amounts of reagents from stocks which creates a major opportunity for contamination.
Methods and compositions for the preparation of bulk quantities of preserved proteins are known, see for example, U.S. Pat. Nos. 5,098,893; 4,762,857; 4,457,916; 4,891,319; 5,026,566 and International Patent Publications WO 89/06542; WO 93/00806; WO 95/33488 and WO 89/00012, all of which are hereby incorporated by reference in their entirety. However, the use of pre-aliquoted and preserved reagent components in single reaction quantities/dose is both very useful and economical. Single aliquots of enzyme reagent avoids multiple use of bulk reagents, reduceing waste, and greatly reducing the chance of contamination. Further, such single reaction aliquots are most suitable for the automation of the reaction process.
The requirement for many changes of buffer and the multiple addition of reagents complicates the automation of such reactions. A single dose unit of reaction buffer mixture, and a unified combination buffer will both simplify automation of the process and reduce the chance of contamination.
Automation of Nucleic Acid Detection with or without Amplification
Nucleic acid probe assays, and combination amplification/probe assays can be rapid, sensitive, highly specific, and usually require precise handling in order to minimize contamination with non-specific nucleic acids, and are thus prime candidates for automation. As with conventional nucleic acid detection protocols, it is generally required to utilize a detection probe oligonucleotide sequence which is linked by some means to a detectable signal generating component. One possible probe detection system is described in U.S. Pat. No. 4,581,333 hereby incorporated by reference in its entirety.
In addition, automation of a nucleic acid detection system targeting unamplified or amplified nucleic acid, or a combined automated amplification/detection system will generally be adaptable to the use of nucleic acid capture oligonucleotides that are attached to some form of solid support system. Examples of such attachment and methods for attachment of nucleic acid to solid support are found in U.S. Pat. Nos. 5,489,653 and 5,510,084 both of which are hereby incorporated by reference.
Automation of amplification, detection, and a combination of amplification and detection is desirable to reduce the requirement of multiple user interactions with the assay. Apparatus and methods for optically analyzing test materials are described for example in U.S. Pat. No. 5,122,284 (hereby incorporated by reference in its entirety). Automation is generally believed to be more economical, efficient, reproducible and accurate for the processing of clinical assays. Thus with the superior sensitivity and specificity of nucleic acid detection assays, the use of amplification of nucleic acid sequences, and automation at one or more phases of a assay protocol can enhance the utility of the assay protocol and its utility in a clinical setting.
Advantage of Internal Control Sequences
Nucleic acid amplification is highly sensitive to reaction conditions, and the failure to amplify and/or detect any specific nucleic acid sequences in a sample may be due to error in the amplification process as much as being due to absence of desired target sequence. Amplification reactions are notoriously sensitive to reaction conditions and have generally required including control reactions with known nucleic acid target and primers in separate reaction vessels treated at the same time. However, internal control sequences added into the test reaction mixture would truly control for the success of the amplification process in the subject test reaction mixture and would be most useful. U.S. Pat. No. 5,457,027 (hereby incorporated by reference in its entirety) teaches certain internal control sequences which are useful as an internal oligonucleotide standard in isothermal amplification reactions for Mycobacterium tuberculosis.
However it would be extremely useful to have a general method of generating internal control sequences, that would be useful as internal controls of the various amplification procedures, which are specifically tailored to be unaffected by the nucleic acid sequences present in the target organism, the host organism, or nucleic acids present in the normal flora or in the environment. Generally, such internal control sequences should not be substantially similar to any nucleic acid sequences present in a clinical setting, including human, pathogenic organism, normal flora organisms, or environmental organisms which could interfere with the amplification and detection of the internal control sequences.
Detection of More than one Nucleic Acid Sequence in a Single Assay
In general, a single assay reaction for the detection of nucleic acid sequences is limited to the detection of a single target nucleic acid sequence. This single target limitation increases costs and time required to perform clinical diagnostic assays and verification control reactions. The detection of more than one nucleic acid sequence in a sample using a single assay would greatly enhance the efficiency of sample analysis and would be of a great economic benefit by reducing costs, for example helping to reduce the need for multiple clinical assays.
Multiple analyte detection in a single assay has been applied to antibody detection of analyte as in for example International Patent Publication number WO 89/00290 and WO 93/21346 both of which are hereby incorporated by reference in their entirety.
In addition to reducing cost, time required, the detection of more than one nucleic acid target sequence in a single assay would reduce the chance of erroneous results. In particular multiple detection would greatly enhance the utility and benefit using internal control sequences and allow for the rapid validation of negative results.